Corrosion-resistant alloy



United States Patent 3,135,603 CORROSION-RESISTANT ALLQY Stanley T. Wlodek, Cincinnati, Ohio, assignor to Union Carbide Corporatioma corporation of New York No Drawing. Filed Sept. 14, 1961, Ser. No. 137,961 1 Claim. (Cl. 75-170) This invention relates to nickel-columbium-silicon alloys characterized by high resistance to corrosion by oilash.

Oil-ash corrosion is corrosion resulting from contact of vanadium pentoxide in an oxidizing environment with a metal surface. The many iron-, nickel-, and cobalt-base alloys typify metals which are subject to this regenerative, catastrophic, linear oxidation. Corrosion of this type is prevalent where metals of this type are in contact with the products of combustion of fuels containing vanadium pentoxide such as oil-burning marine boilers and turbines.

This problem can be avoided, of course, by avoiding vanadium pentoxide-containing fuels in favor of higher grade fuels. However, it would be very desirable to be able to utilize the cheaper fuel oils, and much research has gone into determining means of minimizing oil-ash corrosion. Attempts to solve the problems have included the use of addition agents in the fuel to stabilize the pentoxide as a stable vanadate, and the use of similar constituents as alloying elements with the hope that stabilizing vanadates might form on the alloy during exposure to the corrosive atmosphere. To date, none of these has proved truly effective.

Attempts to develop ash-resistant alloys have shown that high silicon alloys possess improved corrosion resistance. However, the resistance is obtained only by sacrificing ductility in the use of large amounts of silicon.

Typical analyses of oil ashes, as well as a more detailed discussion of the problems and proposed solutions relating to oil ash corrosion are found in Metailurgia, vol. 57, pp. 123-136, 167-172, and 224232 (1958). It should be noted that the conclusions drawn in the paragraph bridging pages 230 and 231 of the Metallurgia study indicate that there are still serious limitations on the use of residual oil.

It is an object of this invention to provide an alloy having high resistance to corrosion by vanadium pentoxide-containing environments while maintaining suitable resistance to mechanical and thermal shock.

It is another object of this invention to provide a means for improving the resistance of predominantly nickel alloys to oil ash corrosion.

Still another object of the invention is to provide fabricated articles which may be employed at elevated temperatures in contact with the products of combustion of vanadium-contaminated fuels.

Other objects will be apparent from the subsequent disclosure and appended claim.

Elemental nickel and predominantly nickel alloys possess inherent resistance to a Wide variety of corrosive media, but these cannot withstand the catastrophic corrosion of vanadium pentoxide under the conditions encountered in fuel-ash corrosion. The most resistant commercial nickel-silicon alloys available have such a high silicon content as to render processing of the alloy as a wrought product impractical.

It has been found that the addition of from 8 to 12 weight percent of columbium to nickel and to predom nantly nickel alloys (alloys containing about weight percent of nickel and higher) greatly improves the resistance of the metals to corrosion by vanadium pentoxide. The alloy consisting essentially of 8 to 12 weight percent columbium, 0.5 to 4 weight percent silicon, and the balance nickel and incidental impurities exhibits exceptional resistance to fuel ash corrosion while being sufliciently ductile to permit easy fabrication. Marine boilers, turbine blades, heat exchangers in closed-cycle turbines and like objects for use in contact with the products of combustion of vanadium-contaminated fuel oil are suitably fabricated from columbium-containing nickel alloys and exhibit long life in the corrosive atmosphere.

The alloy of the present invention was studied under severe simulated service conditions. A vanadium-containing fuel ash was simulated by an 80 percent V O 20 percent Na SO mixture which is one of the most corrosive vanadate-suifate combinations. This mixture was prepared from chemically pure Na SO and commercially pure V 0 (99.9 percent), ball milled to minus 325 mesh, and then moistened slightly with methyl alcohol to allow pressing into l-gram, /2-inch diameter compacts.

The test specimens consisted of a lby 1- by /2-inch rectangular parallelepiped with a /2-inch diameter hole, %lllCh deep, drilled in one of the large faces. By packing this depression with a compact of the simulated oilash mixture, the specimen served simultaneously as the container for the reactants and as the sample being corroded. The prepared specimens were held at 900 C. for 24 and hours in an open horizontal furnace. Alundum boats were used to contain the specimens.

The amount of corrosion that took place was measured by the gain in weight after exposure in air in the presence of the above corrosive mixture. This measurement was supported by a loss in weight value obtained by pickling off the oxides after test in a 20 percent NaOH-S percent KMnO solution. Both thee weight gain and the weight loss so determined were recorded in mg./cm. with the whole sample surface being taken as the affected area.

The results of the tests are presented in Table I.

The data in Table I shows that the addition of columbium and silicon to nickel greatly increases the resistance to oil ash corrosion. It is to be noted, however, that an addition of silicon in excess of 4 percent decreases the ductility of the alloy and increases the rate of oil-ash corrosion. While some silicon is necessary to obtain maximum resistance to corrosion, amounts appreciably in excess of 4 percent must be avoided if weak and brittle alloys are to be avoided.

The alloys of Table I were prepared from commercial purity materials which were compacted and then melted into a IOU-gram button under argon by means of a nonconsumable tungsten arc furnace. Other methods and procedures familiar to skilled metallurgists are permissible there being no intention in the above description to limit the practice in any way.

The new nickel-base alloys of the present invention 10 4 What is claimed is: An alloy consisting essentially of 8 to 12 weight percent columbium, 0.5 to 4 weight percent silicon, and the balance nickel.

References Cited in the file of this patent UNITED STATES PATENTS 2,266,318 Heller Dec. 16, 1941 2,576,123 Kihlgren et al Nov. 27, 1951 3,015,880 Stephenson Jan. 9, 1962 FOREIGN PATENTS 722,405 Great Britain Ian. 26, 1955 762,545 Great Britain Nov. 28, 1956 

